1. Field of the Disclosure
The present disclosure relates generally to geological exploration in wellbores. More particularly, the present disclosure describes an apparatus, a machine-readable medium, and a method useful for obtaining improved resistivity measurements downhole.
2. Description of the Related Art
A variety of techniques are currently utilized in determining the presence and estimating quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including, among other things, the resistivity, porosity, and permeability of a rock formation surrounding a wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. Moreover, a significant amount of the logging, such as in the case of resistivity measurements of the formation, is done after the wellbores have been cased. Typically the casing is made of an electrically conductive metal.
The oil industry has long sought to measure resistivity through casing. Such resistivity measurements are useful for at least the following purposes: locating bypassed oil and gas; reservoir evaluation; monitoring water floods; measuring quantitative saturations; cement evaluation; permeability measurements; and measurements through a drill string attached to a drilling bit. Therefore, measurements of resistivity through metallic pipes, and steel pipes in particular, are an important subject in the oil industry. See, for example, U.S. Pat. No. 4,820,989; U.S. Pat. No. 4,882,542; U.S. Pat. No. 5,043,688; U.S. Pat. No. 5,043,669; U.S. Pat. No. 5,075,626; U.S. Pat. No. 5,187,440; and U.S. Pat. No. 5,223,794 (Ser. No. 07/754,96). Resistivity measurements of the formation that are conducted through an electrically conductive metal casing present significant challenges. In particular, the very small magnitudes of the measured resistivity signals obtained via current existing galvanic methodologies represent critical problems. When using induced magnetic excitations, the skin effects of the electrically conductive metal casing require that low frequencies be used.
A method for measuring shear wave velocity in a cased borehole is disclosed in U.S. Pat. No. 6,850,168 to Tang et al. A transmitter is operated at a frequency below a cut-off frequency of a signal propagating through the drill collar so that the signal received at receiver is uncontaminated by the collar mode. The cut-off frequency is determined by the thickness of the drill collar. In a preferred embodiment, the transmitter is a quadrupole transmitter. The quadrupole mode in the collar has a cut-off frequency that is higher than the quadrupole mode in the formation, so that operating a quadrupole transmitter below this cut-off frequency gives a signal relatively uncontaminated by the tool mode.
Measurements based on the velocity of acoustic waves through an earth formation in the presence of an applied magnetic field can yield useful information concerning a parameter of the earth formation. As the wellbore casing is generally electrically conductive, there is a need for addressing the presence of the casing in electrical measurements.